Key Steps in Nucleotide Biosynthesis Are Regulated by Feedback Inhibition

III. Synthesizing the Molecules of Life
25. Nucleotide Biosynthesis
25.3. Deoxyribonucleotides Synthesized by the Reduction of Ribonucleotides Through a Radical Mechanism
Figure 25.15. Suicide Inhibition. Fluorodeoxyuridylate (generated from fluorouracil) traps thymidylate synthase in a
form that cannot proceed down the reaction pathway.
III. Synthesizing the Molecules of Life
25. Nucleotide Biosynthesis
25.4. Key Steps in Nucleotide Biosynthesis Are Regulated by Feedback Inhibition
Nucleotide biosynthesis is regulated by feedback inhibition in a manner similar to the regulation of amino acid
biosynthesis (Section 24.3). Indeed, aspartate transcarbamoylase, one of the key enzymes for the regulation of
pyrimidine biosynthesis in bacteria, was described in detail in Chapter 10. Recall that ATCase is inhibited by CTP, the
final product of pyrimidine biosynthesis, and stimulated by ATP. Carbamoyl phosphate synthetase is a site of feedback
inhibition in both prokaryotes and eukaryotes.
The synthesis of purine nucleotides is controlled by feedback inhibition at several sites (Figure 25.16).
1. The committed step in purine nucleotide biosynthesis is the conversion of PRPP into phosphoribosylamine by
glutamine phosphoribosyl amidotransferase. This important enzyme is feedback-inhibited by many purine
ribonucleotides. It is noteworthy that AMP and GMP, the final products of the pathway, are synergistic in inhibiting the
amidotransferase.
2. Inosinate is the branch point in the synthesis of AMP and GMP. The reactions leading away from inosinate are sites
of feedback inhibition. AMP inhibits the conversion of inosinate into adenylosuccinate, its immediate precursor.
Similarly, GMP inhibits the conversion of inosinate into xanthylate, its immediate precursor.
3. As already noted, GTP is a substrate in the synthesis of AMP, whereas ATP is a substrate in the synthesis of GMP.
This reciprocal substrate relation tends to balance the synthesis of adenine and guanine ribonucleotides.
The reduction of ribonucleotides to deoxyribonucleotides is precisely controlled by allosteric interactions. Each
polypeptide of the R1 subunit of the aerobic E. coli ribonucleotide reductase contains two allosteric sites: one of them
controls the overall activity of the enzyme, whereas the other regulates substrate specificity. The overall catalytic activity
of ribonucleotide reductase is diminished by the binding of dATP, which signals an abundance of deoxyribonucleotides.
The binding of ATP reverses this feedback inhibition. The binding of dATP or ATP to the substrate-specificity control
sites enhances the reduction of UDP and CDP, the pyrimidine nucleotides. The binding of thymidine triphosphate (TTP)
promotes the reduction of GDP and inhibits the further reduction of pyrimidine ribonucleotides. The subsequent increase
in the level of dGTP stimulates the reduction of ATP to dATP. This complex pattern of regulation supplies the
appropriate balance of the four deoxyribonucleotides needed for the synthesis of DNA.